The invention relates to a multi-user communication system comprising at least a base transceiver station and a plurality of user transceiver stations communicating with one other over transmission channels via the base station, the system comprising:
transforming means for transforming transmit messages into symbols of a constellation at the transmitter end and means for performing reverse options in each user station at the receiver end, PA1 coding means for coding said symbols with a code by spread spectrum coding at the transmitter end and decoding means for performing reverse operations at the reveiver end, PA1 means for transmitting/receiving coded data over a channel. PA1 input serial/parallel converter means for coded transmit symbols, PA1 means for simultaneously applying an inverse Fourier transform to the symbols to be transmitted on the plurality of carriers, PA1 means for adding protection symbols against echoes, PA1 parallel/serial converter means, PA1 means for forming frames of symbol blocks and for adding special symbol blocks. PA1 a sampler PA1 input serial/parallel converter means for received modulated data, PA1 means for applying a direct Fourier transform to produce said received coded symbols, PA1 parallel/serial converter means.
The invention likewise relates to a user transceiver station, more specifically, either a mobile station ensuring communication by radio links, or a fixed station communicating by cables, satellites or otherwise. The invention likewise relates to a base station intended and relay messages with the object of putting mobile stations into communication with one another.
A communication system among users is to satisfy several criteria and among these criteria the criterion of confidentiality is easily satisfied. The designers of such systems are thus oriented at the use of codes for the transmission of transmit messages. A high-performance coding system is the Code Division Multiple Access CDMA system. This system presents advantages of confidentiality, robustness to disturbance, robustness to interference or degradation and can furthermore be easily dimensioned to any number of users. The principle of the CDMA system consists of spreading the frequency spectrum. Therefore, a message to be coded, formed by bits of elementary length Ts, is multiplied before transmission by a pseudo-random bit sequence having an elementary length Tc, so that Tc=Ts/L. The parameter L is the period of the sequence, that is to say, the number of bits of the sequence. Each utilizer has thus his own allocated sequence. A base station receives the coded messages coming from all the users, decodes them and then recodes them and relays them to all the users. The base station generates the communications, that is to say, it enables a user and that user alone to decode a message intended for him. Such a system makes the use of an equalizer necessary when receiving, which equalizer may become complex with a rising number of users. In effect, when echoes from a multipath environment occur, the echoes relating to a user's channel will generally not be the same as the echoes from another user's channel. The real performance of such a system is thus limited. On the other hand, it is necessary to synchronize the receiver with the beginning of each sequence, so that the received message can be decoded, which often forms a problem.
Such a system is described, for example, in the book by J. G. PROAKIS, "Digital Communications", chapter 8, pp. 800-817, published by McGraw-Hill Book Company (1989), New York.